T29 Recent Advances in Soil and Paleosol Science (Posters)

Sunday, 9 October 2022
9:00 AM - 1:00 PM
Exhibit Hall F

WATER QUALITY AND EROSION RATES OF LANDSCAPE CHANGE IN TWO WATERSHEDS, SOUTHEASTERN PUERTO RICO

Paper number: T29-3
Program: Technical Sessions
Time: 9:00 AM – 1:00 PM

Authors

Abstract

To establish baseline behavior and place human impact in context, a Puerto Rican/American team characterized the flux of mass through two adjacent watersheds in southeastern Puerto Rico using geochemical and isotopic data. The Río Guayanés (68.4 km2) and Río Maunabo (13.9 km2) have USGS gauging stations with 20 and 44 years of record, respectively. Although the bedrock underlying the basins is similar (granitic), topography differs between the watersheds.

In March and June 2022, we collected 15 samples from the main stem and tributaries along each river. We measured pH, dissolved oxygen, and conductivity. We used ICP-OES to quantify Ca, K, Mg, and Na in water samples, and IC to measure anions. Silica was determined colorimetrically. We measured E. coli in the field and total dissolved N and organic C in the lab. We extracted in situ 10Be from river sand and determined erosion rates at the basin scale.

The two watersheds have different geochemical, isotopic, and hydrologic characteristics. Unit area run-off is higher in the Maunabo (0.040 cms/km2) than the Guayanés Basin (0.031 cms/km2) reflecting greater mean annual precipitation in the Maunabo (2.0 m/yr) than the Guayanés (1.7 m/yr). Erosion rates, calculated from in situ 10Be concentrations in quartz, are higher in the Maunabo (67+/-11 m/My) than the Guayanes Basin (40+/-25 m/My, average, 1SD). Conductivity and pH are higher in the Maunabo (219+/-38 uS, 7.8+/-0.1) than the Guayanés Basin (141+/-24 uS, 7.3+/-0.2). Consistent with measured conductivity, concentrations of Ca+2, Mg+, Na+, HCO3-, Cl-, and SO4-2 are higher in Río Maunabo water than water from Río Guayanés. Average concentrations of K+, F+, NO3-, and SiO2 are similar between the basins as are dissolved oxygen and E. coli (97% vs 310 colonies/100 ml) as well as DOC and TDN.

Overall, we infer that the flux of mass per unit area from the Maunabo Basin is greater than from the Guayanés Basin. We make this inference based both on dissolved load and 10Be- based rates of erosion at the basin scale. This implies differences in the integrity and composition of the underlying bedrock. In contrast, concentrations of materials more likely to be influenced by human actions are similar between the two basins (dissolved oxygen E. coli, DOC, TDN and NO3- ) suggesting, as field and air photo observations show, similar levels of human impact.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-382542

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